The present invention relates to a polishing pad and to a method of polishing semiconductor substrates where this is employed and, furthermore, it relates to a polishing pad for mechanically planarizing the surface of the insulating layers and metallic interconnects formed on silicon or other such semiconductor substrates.
Year by year, the mounting densities of large scale integrated circuits (LSIs) typified by semiconductor memories have increased and, along with this, the widths of the interconnects on the large scale integrated circuits have narrowed and the number of superimposed layers has increased. Due to this increase in the number of superimposed layers, while not an issue in the past, unevenness of the semiconductor wafer main face, produced as a result of the layering, has become a problem. As a result, as described in, for example, Nikkei Microdevice, July 1994, pp 50-57, semiconductor wafer planarization using chemical mechanical polishing (CMP) techniques has been investigated with the object of dealing with the inadequate depth of focus at the time of light exposure brought about by the unevenness produced by layering, or with the object of improving interconnect densities by planarizing through-hole regions.
Generally speaking, CMP equipment comprises a polishing head which holds the semiconductor wafer, which is the material being treated, a polishing pad for carrying out the polishing treatment of the material being treated and a polishing platen which holds this polishing pad. In the semiconductor wafer polishing treatment, the wafer surface layers are made smooth by effecting relative motion between the semiconductor wafer and the polishing pad, and removing the projecting portions on the semiconductor wafer surface layers, using a slurry consisting of abrasive particles and chemical liquid. The polishing rate of a semiconductor wafer, for example in the case of a silicon oxide (SiO2) film formed on the main face of a semiconductor wafer, is roughly proportional to the relative speed between semiconductor wafer and polishing pad, and to the load. Thus, in order to carry out uniform polishing of each part of a semiconductor wafer, it is necessary to make the load applied to the semiconductor wafer uniform.
When insulating layers and the like formed on the main face of a semiconductor wafer are subjected to polishing, if the polishing pad is too soft then, the local planarity is adversely affected. For such reasons, at present a foamed polyurethane sheet of Shore A-type hardness not less than 90xc2x0 is employed. However, with foamed polyurethane pads of high hardness, problems have arisen in that the degree of planarity varies between areas of different densities of unevenness of the insulating layers and the like, and a global step height is produced. There is also the problem that dishing (where the height of the central region of a metallic interconnect is lower than the edges) occurs when the width of Damascene-based metallic interconnects is large. Furthermore, there have also been problems in that the polishing agent is readily adsorbed and clogging tends to occur, or permanent set of the pad surface layer region is produced during the polishing, and so the polishing rate decreases.
The objective of the present invention lies in offering a polishing pad where, in the case of a polishing pad for mechanically flattening the surfaces of the insulating layers or metallic interconnects formed on a silicon substrate, the polishing rate is high, the global step height is low, dishing does not readily occur at the metallic interconnects, clogging and permanent set of the surface layer region do not tend to occur, and the polishing rate is stable.
The present invention has the following constitution.
xe2x80x9cA polishing pad of micro rubber A-type hardness at least 80xc2x0, which contains polyurethane and polymer produced from a vinyl compound, and has closed cells of average cell diameter no more than 1000 xcexcm and, furthermore, has a density in the range 0.4 to 1.1.xe2x80x9d
Below, the form for practising the invention is explained.
Firstly, the micro rubber A-type hardness referred to in the present invention denotes the value evaluated using a micro rubber durometer MD-1 produced by the Kobunshi Keiki Co. Ltd. The micro rubber durometer MD-1 enables hardness measurements to be carried out on thin/small items where measurement has been difficult with conventional durometers, and because it has been designed and manufactured as a spring-system rubber durometer A-type model scaled down to approximately ⅕, measured values which are in agreement with the spring-system rubber durometer A-type hardness are obtained. Since the polishing layer or hard layer thickness in the case of ordinary polishing pads extends less than 5 mm, evaluation is not possible with a spring-system rubber A-type durometer and so evaluation is carried out with this micro rubber durometer, MD-1.
For the polishing pad of the present invention, a micro rubber A-type hardness of at least 80xc2x0, and preferably at least 90xc2x0, is necessary. If the micro rubber A-type hardness is not at least 80xc2x0, the planarity of the local unevenness on the semiconductor substrate is unsatisfactory, so this is undesirable.
Since the polishing pad of the present invention has closed cells, it possesses elasticity in the thickness direction and even when slurry aggregates and polishing debris are sandwiched between the surface undergoing polishing and the polishing pad, scratching can be prevented. It is necessary that the closed cell diameter be no more than 1000xcexc, as an average diameter, so that local unevenness is not brought about. No more than 500 xcexcm is preferred, with no more than 30 xcexcm still further preferred.
It is preferred that the polishing pad of the present invention has a density lying in the range 0.4 to 1.1. If the density is not at least 0.4, the local planarity is poor and there is a considerable global step height. If the density exceeds 1.1, scratching readily occurs. It is further preferred that the density lies in the range 0.6 to 0.9, with a density in the range 0.65 to 0.85 still more preferred.
The polyurethane in the polishing pad of the present invention is a polymer obtained from a polyisocyanate and a compound containing active hydrogen, specifically a hydroxy or amino group-containing compound with two or more active hydrogens. As examples of the polyisocyanate, there are tolylene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, tolidine diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate, but there is no restriction thereto. Polyols are typical of the polyhydroxy compounds, examples of which are polyether-polyols, polypropylene glycol, polytetramethylene ether glycol, epoxy resin-modified polyols, polyester polyols, acryl polyols, polybutadiene polyols, silicone polyols and the like. Of these, the polyurethanes obtained from a combination of tolylene diisocyanate or diphenylmethane diisocyanate, as the polyisocyanate, and polypropylene glycol or polytetramethylene ether glycol, as the polyol, are outstanding in their mouldability and are widely used, so are preferred.
The present invention is a polishing pad containing polyurethane and polymer produced from a vinyl compound, and which has closed cells. With a polyurethane, as the hardness is raised it becomes more brittle. Furthermore, while it is possible to raise the toughness and hardness merely using polymer from a vinyl compound, it has been difficult to obtain a homogenous polishing pad with closed cells. However, by incorporating polyurethane and polymer produced from a vinyl compound, it has been possible to produce a polishing pad of high toughness and hardness which contains closed cells.
In the present invention, a vinyl compound means a compound with a polymerizable carbon-carbon double bond. Specific examples are methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, methyl(xcex1-ethyl)acrylate, ethyl(xcex1-ethyl)acrylate, propyl(xcex1-ethyl)acrylate, butyl(xcex1-ethyl)acrylate, 2-ethylhexyl methacrylate, isodecyl methacrylate, n-lauryl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxybutyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, methacrylic acid, glycidyl methacrylate, ethylene glycol dimethacrylate, fumaric acid, dimethyl fumarate, diethyl fumarate, dipropyl fumarate, maleic acid, dimethyl maleate, diethyl maleate, dipropyl maleate, acrylonitrile, acrylamide, vinyl chloride, styrene, xcex1-methylstyrene and the like. Amongst these, the preferred vinyl compounds are methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, methyl(xcex1-ethyl)acrylate, ethyl(xcex1-ethyl)acrylate, propyl(xcex1-ethyl)acrylate and butyl(xcex1-ethyl)acrylate. Polyurethanes are easily impregnated with the aforesaid preferred vinyl compounds, and when the vinyl compound is polymerized within the polyurethane there is obtained a polishing pad of high hardness and high toughness. Specific examples of the polymers produced from the vinyl compounds in the present, invention (below referred to as the vinyl polymers) are polymethyl methacrylate, polyethyl methacrylate, polypropyl methacrylate, poly(n-butyl methacrylate), polyisobutyl methacrylate, polymethyl(xcex1-ethyl)acrylate, polyethyl(xcex1-ethyl)acrylate, polypropyl(xcex1-ethyl)acrylate, polybutyl(xcex1-ethyl)acrylate, poly(2-ethylhexyl methacrylate), polyisodecyl methacrylate, poly(n-lauryl methacrylate), poly(2-hydroxyethyl methacrylate), poly(2-hydroxypropyl methacrylate), poly(2-hydroxyethyl acrylate), poly(2-hydroxypropyl acrylate), poly(2-hydroxybutyl methacrylate), polydimethylaminoethyl methacrylate, polydiethylaminoethyl methacrylate, polymethacrylic acid, polyglycidyl methacrylate, polyethyleneglycol dimethacrylate, polyfumaric acid, polydimethyl fumarate, polydiethyl fumarate, polydipropyl fumarate, polymaleic acid, polydimethyl maleate, polydiethyl maleate, polydipropyl maleate, polyacrylonitrile, polyacrylamide, polyvinyl chloride, polystyrene, poly(xcex1-methylstyrene) and the like. Of these, as preferred polymers, polymethyl methacrylate, polyethyl methacrylate, polypropyl methacrylate, poly(n-butyl methacrylate), polyisobutyl methacrylate, polymethyl(xcex1-ethyl)acrylate, polyethyl(xcex1-ethyl)acrylate, polypropyl(xcex1-ethyl)acrylate and polybutyl(xcex1-ethyl)acrylate can raise the polishing pad hardness, and are tough and can enhance the planarization properties. In the present invention, the vinyl polymer content is preferably at least 50 wt % and no more than 90 wt %. If it is less than 50 wt %, then the hardness of the polishing pad is lowered, which is undesirable. If it exceeds 90 wt %, the pad elasticity is impaired, so this is undesirable. The polyurethane or vinyl polymer content of the polishing pad can be measured by subjecting the polishing pad to pyrolysis gas chromatography/mass spectroscopy. With regard to the equipment employed in this procedure, as an example of the pyrolyzer, there is the Double Shot Pyrolyzer PY-2010D (produced by Frontier Lab Inc.) and as an example of the gas chromatograph/mass spectrometer, there is the TRIO-1 (produced by the VG Co.).
What is meant in the present invention by the polyurethane and the vinyl polymer being integrally incorporated is that they are not incorporated in a state in which the polyurethane phase and the polymer phase derived from the vinyl compound are separate from one-another. Expressed quantitatively, the infrared spectrum of the polishing pad observed with an infrared microspectroscope of spot size 50 xcexcm has the infrared absorption bands of the polyurethane and the infrared absorption bands of the polymer derived from the vinyl compound, and the infrared spectrum is essentially the same in every location. As an example of the infrared microspectroscope here, there is the IRxcexcs produced by the Spectra-Tech Co.
With regard to the method of producing the polishing pad of the present invention, a preferred method is the method whereby a previously-produced foamed polyurethane sheet having closed cells of average cell diameter no more than 1000 xcexcm and having a density in the range 0.1 to 1.0 is swollen with the vinyl compound, after which the vinyl compound is made to polymerize within the foamed polyurethane. In this way, it is possible to produce a polishing pad with a closed cell structure in which the polyurethane and vinyl polymer are integrally coupled, and so it is possible to enhance local planarity and reduce the global step height. In a further preferred method, there is used a foamed polyurethane sheet material of average cell diameter no more than 500 xcexcm and of density in the range 0.4 to 0.9. Of course, the combination and optimum amounts of the polyisocyanate, polyol, catalyst, foam stabilizer and foaming agent need to be determined in accordance with the target polishing pad hardness, cell diameter and foaming expansion factor.
As examples of methods for bringing about polymerisation of the vinyl compound within the foamed polyurethane, there are the method of polymerisation by light exposure using a photo radical initiator, the method of polymerisation by applying heat using a thermal radical initiator, and the method of polymerisation by means of an electron beam or by radiation.
In the polishing pad of the present invention, there can also be included abrasive particles. Examples of the abrasive particles are silica-based abrasive particles, aluminium oxide-based abrasive particles, cerium oxide-based abrasive particles and the like. It is desirable that the abrasive particles be incorporated into the foamed polyurethane beforehand.
The polishing pad obtained in the present invention can be used in the form of a composite polishing pad laminated to a sheet having cushioning properties. On a semiconductor substrate, besides local unevenness, there are also present somewhat larger undulations, and the polishing is most often carried out with a cushioning sheet placed beneath the hard polishing pad (on the polishing machine platen side) as a layer for absorbing these undulations.
Explanation is now given of the method of polishing a semiconductor substrate using the polishing pad of the present invention.
When carrying out polishing with the polishing pad of the present invention, if there is used a silica-based polishing agent, aluminium oxide-based polishing agent, cerium oxide-based polishing agent or the like as the polishing agent, it is possible to locally planarize insulating film or metal interconnect surface unevenness on the wafer and it is possible to reduce the global step height and suppress dishing. The polishing pad of the present invention is fixed to the rotating platen of the polishing machine and the wafer is held on the wafer carrier by means of a vacuum chuck system. The platen is made to rotate, and the wafer carrier is made to rotate in the same direction and pressed against the polishing pad. At this time, polishing agent is supplied between the polishing pad and wafer. The pressing pressure is adjusted by control of the force applied to the wafer carrier. Local planarity is obtained with a pressing pressure of 0.01 to 0.1 MPa, so this is preferred.
The polishing pad of the present invention makes it possible, when planarizing local unevenness on a semiconductor substrate, to reduce the global step height, suppress dishing and raise the polishing rate, and clogging or permanent set at the pad surface do not readily occur and there is no tendency for the polishing rate to deteriorate with elapse of time, so stable polishing is possible.